A tube of the above type that has a flat section is already known in itself. A tube of the above kind has large, flat or curved faces against which are brazed or welded undulating spacers forming heat exchange fins with a view to constituting a bundle formed of a multiplicity of tubes and spacers.
A depression is formed on the surface of the tube, at the place where the margin is folded inwards, the depression corresponding to the fact that the fold is not a right-angled fold but a circular arc. This depression must be filled in, on the one hand to assure continuity of each of the large faces of the tube in the regions near the cross-piece and, on the other hand, to assure a sealed connection between the tubes and the header plates of the heat exchanger.
The problem is to fill the brazing opening properly during fabrication of the tube.